WHO and UNICEF have just published a new evidence brief on solar direct-drive (SDD) vaccine refrigerators and freezers. It includes case studies from Tanzania, Colombia and Kenya, as well as an overview of SDD technology and how to make sure that SDD technology is the right choice. Here's the link:

“Solar direct-drive (SDD) refrigerators and freezers can be a good option for vaccine storage in areas without reliable electricity, and many models are now WHO-prequalified. But with little information on SDD field performance currently available, making a case for investing in this new technology can be problematic. This evidence brief provides supply chain managers in low- and middle-income countries with a summary of how recent SDD projects have performed, highlighting problems encountered and the steps that were taken to resolve them. An overview of how SDD technology works, and how to make sure that SDD technology is the right choice, is also provided.”

It provides a nice overview of SDD projects, but for those looking for more detailed guidance on how to implement successful solar-powered vaccine refrigerator and freezer systems, I would also recommend the following much longer WHO-UNICEF publication:

I would be interested to hear the thoughts of other members on the new evidence brief.

PS. If you’re looking for more information on other SDD projects, check out at the following forum discussion, which includes contributions from members regarding SDD projects in Somalia, Ethiopia, Malawi, and Rwanda.

Many thanks to all for the lively and illuminating replies. I have a brief comment on Larry's point about Figure 1 on page 2 of the evidence brief. ("This figure is misleading. In the illustration the battery powered refrigerator is powered by four solar modules and four batteries. The SDD refrigerator is powered by only three panels. In actuality a battery powered refrigerator could be a single solar panel and the SDD refrigerator would require an array about four times larger.")

Figure 2 is intended to be a general representation only. Its main purpose is to show that solar battery systems necessarily include more components than the simpler SDD systems. Considering the available space and purpose of the figure, it would be impractical to show every possible combination of solar module and battery configuration. On reflection, I think it is the title of the figure (“Differences between battery-powered and SDD refrigerators”) which is not quite right and may open the figure up to misinterpretation. It might be an idea to consider updating this.

I look forward to hearing more about the successes (and failures) of other SDD projects. Fingers crossed, we will continue to see further advances in this technology in the future.

The F-1 was tested in Columbia along with our PB charge controller. The freezer was not PQS listed because at the time of installation PQS standards for SDD freezers were not yet established. We are no longer manufacturing the F-1. However, we can build the PB charge controller for other manufacturers. As I mentioned the PB controller allows useable energy to be collected during cloudy conditions and allows the size of a solar array to be reduced by about a factor of 3. The F-1's and PB charge controller were also successfully tested by the Clinton Health Intiative at 13 test sites in Nigeria.

Thanks for your opinion on the need for combination refrigerator/freezers. As I mentioned, when SDD refrigerators were first introduced SDD refrigerator/freezers were not available and , at that time, WHO began promoting SDD refrigeration -- no freezers. Currently a limited number of SDD refrigerator/freezers are available. However, their cost is almost double the cost of an SDD refrigerator only. A decision on which to purchase would depend on funds available and specific needs.

For a combination refrigerator/freezer perhaps the best choice may be a lithion ion powered refrigerator compartment and a solar direct drive freezer incorporating our PB charge controller. This system could be powered by an array of about 225 watts.

I have not heard of the failures in Columbia. If I had more details I could make a more specific comment.

I am sorry to hear that our President removed the United States from the Paris Accords.

In Colombia 18 years ago aproximately, The Ministry of Health bought various solar refrigerators manufactured by Sun Frost. This equipment had two compartment, one to refrigeration and another to freezing with vertical door but the problem was that the frame was built with agglomerated wood and in the high humidity in the coast and jungle destroyed the equipment. I guess you work to Sun Frost, I would like to know your point of view about this mistake.

About your question: "... could estimate what percent of the installed SDD refrigerators could benefit from an accompanying icepack freezer." I think that all the equipments should be combined with refrigeration and a compartment to freezing at least 8 icepacks in 24 hours.

You mentions that F-1 was one equipment installed in Colombia, this equipment has PQS?, I reviewed the Catalogue PQS and I don´t find any Sun Frost. ¿Could you extend this topic?

The size of the SDD is a little higher than battery, but this really is not significant if you have less causes of failure of the system, the charge regulator and the battery are the main causes of failure (near to 85) of these systems.

I read the WHO unicef publication “Solar Direct – Drive Vaccine – Refrigerators & Freezers.” The publication shows the relative size of the power systems for an SDD refrigerator and battery powered refrigerator, Figure 1. This figure is misleading. In the illustration the battery powered refrigerator is powered by four solar modules and four batteries. The SDD refrigerator is powered by only three panels. In actuality a battery powered refrigerator could be a single solar panel and the SDD refrigerator would require an array about four times larger.

An 80 liter refrigerator would typically consume 12 amp hrs/day in an environment with a 24 hr average ampient temperature of 32 deg C. This is typical design temperature for battery powered systems in the tropics. On a 3.5 hr solar day this refrigerator could be powered by a single 60 watt solar module. With a 50 percent safety factor a 90 watt module could be used. This is typically four times smaller than the array used to power the typical SDD refrigerator. To obtain three days of storage a relatively small 36 amp hr battery would be required. If a 25 percent safety factor is desired, the battery size could be increased to 45 amp hrs. This is the size of a small car battery. The required battery size could be further reduced by increasing the size of the solar panel to 125 watts. This would be a relatively low cost measure. High quality lithium ion batteries contain no lead and can provide reliable operation for more than ten years. Currently in California lithium ion batteries in electric vehicles are required to have a performance guarantee of 250,000 kilometers. In a vehicle batteries are subject to much harsher conditions than when powering a vaccine refrigerator. These conditions include higher charge and discharge rates, vibration and high temperatures. Lithium ion batteries which have had problems are in applications where very high energy density is required such as cell phones and hover boards. In a well designed system incorporating lithium batteries the batteries and the control electronics could be pre-wired and built in next to the cooling system of the refrigerator. In the field only the wires to a single solar panel need be connected. These systems would be much easier to transport and install when compared to the multi-panel SDD system. In addition, the refrigerator would be less bulky because the refrigerator itself can be smaller as a consequence of the space taken up and the weight of the phase change storage materials. If the lithium ion battery fails and an original battery cannot be obtained, it could be replaced by a small car type battery. I anticipate that it is inevitable that high quality lithium ion batteries will be approved by WHO.

Almost all the currently installed battery powered solar refrigerators are a combination refrigerator/freezer and require a larger array than a refrigerator only. Perhaps that is why the battery powered system illustrated in figure 1 has the larger solar array.

In the WHO unicef report the relative cost of SDD and battery driven systems was compared. Figure 2 in the publication illustrated a lower yearly cost for the SDD system. With more realistic figures for the size of the solar power system the cost of a battery powered system would be substantially less than an SDD refrigerator.

Before the introduction of SDD refrigerators WHO discouraged manufacturers from building solar powered refrigerators without ice pack freezers. Initially the technology of SDD refrigerators/freezers was not available. With the introduction of SDD refrigerators WHO promoted this refrigeration-only technology. Perhaps readers with field experience could estimate what percent of the installed SDD refrigerators could benefit from an accompanying icepack freezer.

Battery powered refrigerators require a much smaller solar array because they can collect and store energy at low levels of insolation. Typically if the output of the solar array is less than 70 watts the compressor on an SDD refrigerator will not operate. With a battery powered system useable energy will be collected even at very low light levels. Sun Frost has developed a device we call the “PB Charge Controller.” It allows an SDD refrigerator to run at low levels of insolation. Our F-1 freezer contains this technology and was one of the units tested in Columbia by the Solar Electric Light Fund. These test results were described in the WHO unicef report. The F-1 operated successfully for the past three years on a single PV module. This device is available to manufacturers to increase the efficiency and reliability of their SDD refrigerators and freezers.

The concerns expressed by Wendy are worrisome. I would like to add that problems faced on any new equipment should be imperatively escalated to to PQS secretariat and UNICEF country office and SD. It is important that timely action be taken to ask the supplier to remedy the defect if the equipment is still under warrantee.

Late or absence of communication results in the equipment going beyond warrantee period and the supplier washes off his hands. If case the problem is present in several units then it is important that PQS secretariat takes appropriate action with the supplier and in cases of necessity black list the product for the safety of the EPI programme.

I have come across installations of some cold rooms cases (not PQS) procured by MOH and installed through outsourced agencies, which resulted in incomplete delivery and defects being communicated late. This resulted in a chronic problem rather than a support for the EPI.

I am tempted in this context to suggest that considering the delays between receipt of equipment and its installation in the field (equipment procured by the MoH, and not under CCEOP) the start of the warrantee period should be negotiated with the supplier. As an example I would suggest that the warrantee should start from the date of installation or 3 or six month after delivery, whichever comes first. A country cannot benefit of any warranty if the start date is the date of delivery or landing in the the port.

Zimbabwe recently procured and installed more than 100 SDD fridges (ZLF 100 DC and BLF 100 DC). About 20% of the fridges are still having problems maintaining correct temperatures. Additionally, some of the SureChill equipment had issues with the drainage system -- missing a tray for condensation collection -- causing an excess of condensation inside the refrigerator which loosens the labels on the vaccine vials. Technicians are now going out to all facilities to address these issues.

From this experience, I can echo what Kshem and Rafael have already mentioned -- training on installation is very important; all vaccine programs must have a clear and budgeted maintenance plan for all cold chain equipment; spare parts must be available; and the overall system that allows a technician to do his/her job must be strong enough to ensure vehicles and fuel are available to get a technician to a facility that needs maintenance, that spare parts are available, and health workers know how to perform basic preventive maintenance.

A strong cold chain goes beyond just procuring new equipment; the entire system needs to be considered and appropriately planned for.

The common thinking is to blindly replace the absorption fridges with SDDs. However, in most countries, the rainy season may render difficultthe funtionning of SDDs, especially if there is an intense monsoon season. In those days or weeks one can use the existing absorption refrigerators as backup. Thus the health cewntre may require just around 10% of its current need of gas or kerosene, while ensuring the correct storage of vaccines.

By completing removing the absorption refrigerators by SDD, one may not gain except the space occupied by it.

The experience about solarchill in 2007 and SDD from 2013 has been very positive in Colombia where we have 300 equipments SDD working in different kind of climates and environments, jungle, desert, coast, island, mountain. The best equipment is who have freezer and refrigeraror combined. One topic very important is must have a responsable to take care the equipment and get the training to the people of the comunity, not only to vaccinator.

The refrigerator by absortion and SDD are complemented and each of them works in diferent situations. Both are very useful.

This is a very useful compilation of field expereince and learning for qality intallation and sustainability of SDDs. some of the key issues emerging are : 1) Need for adequate user training, 2) need for proper technician training and 3) ensuring adequate and continuous funding for maintenance.

It should be noted that under the Gavi's CEEOP, the training of users and technicians is included within the bundling of the equpment. The countries would do well to benifit from this at least for the intial set of equipmnent if not all. For this the EPI should identify the technicians and staff to be trained.

As for securing funding and its sustainability for preventive and curative maintenance, all countries switching from the Gas / Kerosene based equipment should compute their annual saving on fuel and its transport (which is now saved for every year for the next decade). The total annual funding required for the preventive and curative maintenance would be just a fraction of this. It would be wise and judicious if they would set aside this budget for the sustainability of the SDDs, while making the big saving from the Gas/kerosene.